Summary: We have initiated studies to investigate the period of viremia post-vaccination. Cells and plasma from recent vaccinnes enrolled in candidate vaccine trials are being collected at various time points 0, 3, 7, 14, 28 and 56 days. Sensitive Taqman and virus culture assays are being developed. In addition, currently licensed viral marker tests used to screen donors will be evaluated for potential cross-raectivity of interference with samples from immunized individuals. We are also investigating CTL epitopes involved in the immune response against vaccinia.